Re: [PATCH 4/4] iommu/io-pgtable-arm: Prepare for TTBR1 usage

[Will Deacon <will@kernel.org>]

On Tue, Aug 20, 2019 at 03:51:45PM +0100, Robin Murphy wrote:
> On 20/08/2019 11:30, Will Deacon wrote:
> > On Mon, Aug 19, 2019 at 07:19:31PM +0100, Robin Murphy wrote:
> > > Now that callers are free to use a given table for TTBR1 if they wish
> > > (all they need do is shift the provided attributes when constructing
> > > their final TCR value), the only remaining impediment is the address
> > > validation on map/unmap. The fact that the LPAE address space split is
> > > symmetric makes this easy to accommodate - by simplifying the current
> > > range checks into explicit tests that address bits above IAS are all
> > > zero, it then follows straightforwardly to add the inverse test to
> > > allow the all-ones case as well.
> > > 
> > > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> > > ---
> > >   drivers/iommu/io-pgtable-arm.c | 7 ++++---
> > >   1 file changed, 4 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> > > index 09cb20671fbb..f39c50356351 100644
> > > --- a/drivers/iommu/io-pgtable-arm.c
> > > +++ b/drivers/iommu/io-pgtable-arm.c
> > > @@ -475,13 +475,13 @@ static int arm_lpae_map(struct io_pgtable_ops *ops, unsigned long iova,
> > >   	arm_lpae_iopte *ptep = data->pgd;
> > >   	int ret, lvl = ARM_LPAE_START_LVL(data);
> > >   	arm_lpae_iopte prot;
> > > +	long iaext = (long)iova >> data->iop.cfg.ias;
> > >   	/* If no access, then nothing to do */
> > >   	if (!(iommu_prot & (IOMMU_READ | IOMMU_WRITE)))
> > >   		return 0;
> > > -	if (WARN_ON(iova >= (1ULL << data->iop.cfg.ias) ||
> > > -		    paddr >= (1ULL << data->iop.cfg.oas)))
> > > +	if (WARN_ON((iaext && ~iaext) || paddr >> data->iop.cfg.oas))
> > 
> > I had to read that '&&' twice, but I see what you're doing now :)
> > 
> > >   		return -ERANGE;
> > 
> > This doesn't seem sufficient to prevent a mixture of TTBR1 and TTBR0
> > addresses from being mapped in the same TTBR. Perhaps we need a quirk for
> > TTBR1, which could then take care of setting EPDx appropriately?
> 
> Right, that's the one downside of going for the minimalist "io-pgtable
> doesn't even have to know" approach. On reflection, though, in that paradigm
> it should probably be the caller's responsibility to convert TTBR1 addresses
> to preserve the "as if TTBR0" illusion anyway :/

Right, and I'd rather not push stuff into the caller for the common case.
It's not exactly onerous to support this in io-pgtable. It's also why I'd
still like to keep the EPDx in there, because the callers that care can
rewrite the stuff, but at least we provided a default.

> The advantage of not having a quirk is that it allows split address spaces
> to fit more closely with the aux_domain idea, i.e. we could allocate and
> initialise a domain without having to assume, or even care, whether it will
> end up attached as a primary or aux domain. It *might* even be potentially
> useful to have a domain attached to TTBR0 of one device's context and TTBR1
> of another's at the same time, although that's pretty niche.

That sounds pretty theoretical to me at the moment.

Will
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